Device-to-device, D2D, communications in the cellular spectrum is a relatively new concept that targets scenarios in which communicating parties are in the close proximity of each other, see G. Fodor et al, “Design Aspects of Network Assisted Device-to-Device Communications”, IEEE Communications Magazine, March 2012. An advantage with using D2D communication is that the capacity of a radio communication network as a whole is increased without a corresponding impact on cellular infrastructure. Thus, the infrastructure radio communication network may be offloaded in terms of traffic between wireless devices.
Moreover, D2D communication enables infrastructure-less communication between wireless devices. This may be of importance in, for example, emergency, national security and public safety situations, since during these situations load on the radio communication network(s) is generally high. Furthermore, an emergency situation may for example occur where only limited coverage by the radio communication system is provided. In such a situation, D2D communication may improve coverage by allowing wireless devices within an area to communicate with each other. In addition, local communication between wireless devices using D2D communication is achievable without a need for radio coverage by the radio communication system or in general, the radio coverage of a cellular infrastructure independently whether the infrastructure comprises one radio access technology, RAT, or a plurality of RATs.
Recently, D2D communications as a complement to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, it is suggested that device-to-device communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated spectrum for D2D purposes is a less likely alternative as spectrum is a scarce resource and dynamic sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency.
D2D devices that want to communicate, or even just discover each other, typically need to transmit various forms of control signaling. One example of such control signaling is the so-called discovery message, which at least carries some form of identity and is transmitted by a device that wants to be discoverable by other devices. Other devices can scan for the discovery message. Once the D2D devices have detected the discovery message, they can take the appropriate action, for example to try to initiate a connection setup with the device transmitting the discovery message.
Some D2D devices can transmit control signaling, i.e. discovery messages as well as other types of control signaling, simultaneously. The transmissions from the different devices may be time synchronized i.e. mutually time-aligned, or unsynchronized. Synchronization could be obtained for example by receiving appropriate signals from the overlaid cellular network, or from a global navigation satellite system such as GPS. According to ProSe requirements as specified in see 3GPP SA1 ProSe technical report, TR 22.803 V12.2.0 (2013-06) user equipments belonging to one cell need to be able to discover user equipments camping on another cell.
Another scenario arises when user equipments are in physical proximity, but camp on different public land mobile networks, PLMNs, or different carriers, because, while discovery messages transmitted within a cell are typically synchronized to a cell-specific reference, different cells/carriers/PLMNs networks are in general unsynchronized. In other words, from a D2D communication perspective, there are no cell boundaries and user equipments need to track multiple synchronization references.
The above mentioned ProSe technical report recommends supporting D2D operation for out of network coverage user equipments as well as for belonging to one cell. In such case, different synchronization options are possible: user equipments may synchronize to a global reference e.g. a GPS, which is in general different from the synchronization reference of deployed networks. Possibly user equipments may also operate in a fully asynchronous fashion without any synchronization reference, at least for discovery. A further option is that clusters of user equipments synchronize to a specific user equipment, in the following called as Cluster Head, CH, which provides local synchronization to its neighbor user equipments. Different clusters are not necessarily synchronized.
To convey information about out of network D2D devices, user equipments may discover D2D signals with an arbitrary synchronization on a given carrier (or sub band) by searching for D2D signals in time. This can be done, e.g. by time domain correlation of the received signal with the D2D signal waveforms, similarly to the way user equipments search for cells using the primary and secondary synchronization signals, PSS, SSS as defined in the LTE standard.
User equipments alternate wake up and sleep cycles for reducing power consumption, referred to as discontinuous reception, DRX. During sleep periods, only the memory and clocks are active, but the user equipment is unable to receive any signal. During wake up time, the receiver is on. It is essential that the wake up times are as narrow as possible compared to the sleep time in order to save battery.
However, searching for D2D signals by scanning the radio environment entails high energy consumption at the receiver, since the receiver needs to be kept active or awake over a prolonged time. Furthermore, in such a system resource allocation and radio resource management algorithms are problematic because resources used by neighbor D2D user equipments may potentially interfere strongly with the user equipments camping on the cell.
Hence, there is a need for radio resource management and interference mitigation methods overcoming the above mentioned problems to further improve the advantages of D2D communications.